Dense gas in low-metallicity galaxies

Abstract

Stars form out of the densest parts of molecular clouds. Far-IR emission can be used to estimate the Star Formation Rate (SFR) and high dipole moment molecules, typically HCN, trace the dense gas. A strong correlation exists between HCN and Far-IR emission, with the ratio being nearly constant, over a large range of physical scales. A few recent observations have found HCN to be weak with respect to the Far-IR and CO in subsolar metallicity (low-Z) objects. We present observations of the Local Group galaxies M33, IC10, and NGC6822 with the IRAM 30meter and NRO 45m telescopes, greatly improving the sample of low-Z galaxies observed. HCN, HCO${}^{+}$, CS, C${}_2$H, and HNC have been detected. Compared to solar metallicity galaxies, the Nitrogen-bearing species are weak (HCN, HNC) or not detected (CN, HNCO, N${}_2$H${}^{+}$) relative to Far-IR or CO emission. HCO${}^{+}$ and C${}_2$H emission is normal with respect to CO and Far-IR. While ${}^{13}$CO is the usual factor 10 weaker than ${}^{12}$CO, C${}^{18}$O emission was not detected down to very low levels. Including earlier data, we find that the HCN/HCO${}^{+}$ ratio varies with metallicity (O/H) and attribute this to the sharply decreasing Nitrogen abundance. The dense gas fraction, traced by the HCN/CO and HCO${}^{+}$/CO ratios, follows the SFR but in the low-Z objects the HCO${}^{+}$ is much easier to measure. Combined with larger and smaller scale measurements, the HCO${}^{+}$ line appears to be an excellent tracer of dense gas and varies linearly with the SFR for both low and high metallicities.

arXiv comment9 pages, 13 figures, A&A in press

References in corpus (46)

• Dwarf Galaxies of the Local Group
• From filamentary clouds to prestellar cores to the stellar IMF: Initial highlights from the Herschel Gould Belt survey
• The Star Formation Rate and Dense Molecular Gas in Galaxies
• On the Star Formation Rates in Molecular Clouds
• HCN Survey of Normal Spiral, IR-luminous and Ultraluminous Galaxies
• The Star Formation Rate and Gas Surface Density Relation in the Milky Way: Implications for Extragalactic Studies
• The Calibration of Monochromatic Far-Infrared Star Formation Rate Indicators
• Connecting Dense Gas Tracers of Star Formation in our Galaxy to High-z Star Formation
• Star Formation Rates in Molecular Clouds and the Nature of the Extragalactic Scaling Relations
• A multi-transition HCN and HCO+ study of 12 nearby active galaxies: AGN versus SB environments
• The Properties of Massive, Dense, Clumps: Mapping Surveys of HCN and CS
• Abundances of metal-rich H II regions in M51
• Oxygen and Nitrogen in Isolated Dwarf Irregular Galaxies
• Spatially Resolved Chemistry in Nearby Galaxies I. The Center of IC 342
• Star formation laws in Luminous Infrared Galaxies. New observational constraints on models
• A line confusion limited millimeter survey of Orion KL (I): sulfur carbon chains
• Variations in the Star Formation Efficiency of the Dense Molecular Gas across the Disks of Star-Forming Galaxies
• Molecular and Atomic Gas in the Local Group Galaxy M33
• Evidence of enhanced star formation efficiency in luminous and ultraluminous infrared galaxies
• Dense gas in luminous infrared galaxies
• Calibration of the total infrared luminosity of nearby galaxies from Spitzer and Herschel bands
• The EMPIRE Survey: Systematic Variations in the Dense Gas Fraction and Star Formation Efficiency from Full-Disk Mapping of M51
• Enhanced Dense Gas Fraction in Ultra-Luminous Infrared Galaxies
• Molecular Gas in the Low Metallicity, Star Forming Dwarf IC 10
• HCN Observations of Dense Star-Forming Gas in High Redshift Galaxies
• The metallicity profile of M31 from spectroscopy of hundreds of HII regions and PNe
• Dust heating sources in galaxies: the case of M33 (HERM33ES)
• The distance of M33 and the stellar population in its outskirts
• The planetary nebula population of M33 and its metallicity gradient: A look into the galaxy's distant past
• Excitation Mechanisms for HCN (1-0) and HCO+ (1-0) in Galaxies from the Great Observatories All-sky LIRG Survey
• High abundance ratio of ${}^{13}$CO to C${}^{18}$O toward photon-dominated regions in the Orion-A giant molecular cloud
• On the origin of nitrogen
• The Stellar Population and Star Clusters in the Unusual Local Group Galaxy IC 10
• HCN and HCO+ emission in the disk of M31
• Spectral Line Survey toward Spiral Arm of M51 in the 3 mm and 2 mm Bands
• HCN and HNC mapping of the protostellar core Cha-MMS1
• IC10: the history of the nearest starburst galaxy through its Planetary Nebula and HII region populations
• The Molecular Interstellar Medium of the Local Group Dwarf NGC6822
• Spatially-Resolved Dense Molecular Gas and Star Formation Rate in M51
• The Galactic Census of High- and Medium-mass Protostars. II. Luminosities and Evolutionary States of a Complete Sample of Dense Gas Clumps
• Dense Gas in M33 (HerM33es)
• The Green Bank Telescope Maps the Dense, Star-Forming Gas in the Nearby Starburst Galaxy M82
• Spectral Line Survey toward a Molecular Cloud in IC10
• Spectral-Line Survey at Millimeter and Submillimeter Wavelengths toward an Outflow-Shocked Region, OMC 2-FIR 4
• From Gas to Stars in Energetic Environments: Dense Gas Clumps in the 30 Doradus Region Within the Large Magellanic Cloud
• The Radio Continuum, Far-Infrared Emission, And Dense Molecular Gas In Galaxies

Cited by in corpus (26)

• EMPIRE: The IRAM 30-m Dense Gas Survey of Nearby Galaxies
• Testing the universality of the star formation efficiency in dense molecular gas
• Dense gas is not enough: environmental variations in the star formation efficiency of dense molecular gas at 100pc scales in M51
• The MALATANG Survey: the L_gas-L_IR correlation on sub-kiloparsec scale in six nearby star-forming galaxies as traced by HCN J=4-3 and HCO^+ J=4-3
• The headlight cloud in NGC 628: An extreme giant molecular cloud in a typical galaxy disk
• The star-formation law at GMC scales in M33, the Triangulum Galaxy
• The ALMOND Survey: Molecular cloud properties and gas density tracers across 25 nearby spiral galaxies with ALMA
• A molecular line survey toward the nearby galaxies NGC 1068, NGC 253, and IC 342 at 3 mm with the Nobeyama 45 m radio telescope: The data
• Dense-gas tracers and carbon isotopes in five 2.5<z<4 lensed dusty star forming galaxies from the SPT SMG sample
• Planck's Dusty GEMS. VIII. Dense-gas reservoirs in the most active dusty starbursts at z~3
• The MALATANG Survey: Dense Gas and Star Formation from High Transition HCN and HCO+ maps of NGC253
• Dense Molecular Gas in the Nearby Low Metallicity Dwarf Starburst Galaxy IC 10
• An ALMA Glimpse of Dense Molecular Filaments Associated with High-mass Protostellar Systems in the Large Magellanic Cloud
• DeGaS-MC: Dense Gas Survey in the Magellanic Clouds I -- An APEX survey of HCO+ and HCN(2-1) toward the LMC and SMC
• The Evolution of NGC 7465 as Revealed by its Molecular Gas Properties
• Tracers of Dense Gas in the Outer Galaxy
• CON-quest II. Spatially and spectrally resolved HCN/HCO+ line ratios in local luminous and ultraluminous infrared galaxies
• Dense gas and star formation in the Outer Milky Way
• Dense Gas and Star Formation in Nearby Infrared Bright Galaxies: APEX survey of HCN and HCO+ J=2-1
• Dense gas and star formation in individual Giant Molecular Clouds in M31
• Dense gas scaling relations at kiloparsec scales across nearby galaxies with the ALMA ALMOND and IRAM 30m EMPIRE surveys
• Down but not out: properties of the molecular gas in the stripped Virgo Cluster early-type galaxy NGC4526
• A 260 pc resolution ALMA map of HCN(1-0) in the galaxy NGC 4321
• The magnetic field in the Flame nebula
• Tracing circumnuclear dense gas in H2O maser galaxies
• Weak CS Emission in an Extremely Metal-poor Galaxy DDO 70